The RMT has written to Prime Minister Theresa May, accusing her of "sabotaging" negotiations aimed at resolving the disputes.

General secretary Mick Cash said: "RMT members are standing absolutely rock-solid and united this morning in these four separate disputes as they make the case for safe, secure and accessible rail service for all.

"The public and workplace support right across the country for our train guards is nothing short of fantastic as we mount picketing operations at all key locations.

"These strikes are about safety. They are about putting the safety and security of the travelling public before the profits of private companies, profits that in the most part are shipped overseas to subsidise transport services in Europe. That is a national scandal.

"It is disgraceful that we have been unable to get serious and meaningful talks going in any of these four disputes and that points clearly to the dead-hand of the Government driving the faceless railway policy on purely ideological grounds and demanding that their contractors fall in line."

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The Merseyrail strike coincided with improvement work at Liverpool Lime Street station.

Merseyrail managing director Jan Chaudhry-van der Velde said: "At a time when passenger journeys are temporarily restricted due to these important investment works, the RMT has now targeted these as an opportunity to double up the inconvenience for passengers by staging two further strikes.

"So while the industry and key players in the city region work hard to improve the railway, the RMT seems to be doing everything in its power to stop it. Doesn't the RMT want the Liverpool City Region to succeed?"

Paul Cherpeau, chief executive of Liverpool and Sefton Chamber of Commerce, said: "This dispute is causing substantial disruption to businesses in the city region. It has severely impacted upon our most prominent events in the social calendar and is now impeding the ability of employees and visitors to travel into Liverpool by train.

"We urge all sides to resolve the dispute as quickly as possible to ensure that Liverpool remains open for business."

Greater Anglia is planning to run a full service on the strike days and Southern said there will be a normal service on most of its routes.